Eye piece and tunable chromatic dispersion compensator using the same

ABSTRACT

An eye piece ( 403 ) for use in a tunable chromatic dispersion compensator comprises a first strip ( 4031 ) made of a first metal, a second strip ( 4032 ) made of a second metal, a heater/cooler ( 4033 ), and a tunable positioning bar ( 4034 ). The second strip ( 4032 ) is attached to the first strip ( 4031 ), the heater/cooler ( 4033 ) is attached to the second strip ( 4032 ) for heating/cooling the first and second strips ( 4031, 4032 ), the tunable positioning bar ( 4034 ) is attached to the heater/cooler ( 4033 ) for keeping the position of the eye piece ( 403 ) in the tunable chromatic dispersion compensator, and the first metal and the second metal have different expanding coefficients from each other, so that in response to a change in temperature, the shape of the eye piece ( 403 ) is changed from a first shape to a second shape. A tunable chromatic dispersion compensator uses the eye piece ( 403 ).

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to the optical networking field, and inparticular to an eye piece and a tunable chromatic dispersioncompensator using the eye piece and Planar Lightwave Circuit (PLC)devices, and in the chromatic dispersion compensator, the eye piece iscritical for the tunability.

2. Description of Prior Art

Chromatic dispersion compensation is necessary for optical networks; andtunability is the key for reconfigurable or agile networks. Currenttunable methods cannot compensate the chromatic dispersion slope of theoptical fiber over a specific wavelength range (e.g. C band), which isan important problem for DWDM systems, though the amount of chromaticdispersion can be tuned.

Conventional chromatic dispersion compensators use specially designedoptical fibers, which are expensive, and can not be tuned. Nextgeneration optical networks need cheaper and agile devices, and thustunability and reconfigurability are desirable. Tunable dispersioncompensation is one of the blocking factors for realizing agility ofnext generation optical networks. Some tunable dispersion compensatingmodules (DCMs) e.g. Fiber Bragg Grating (FBG) and etalon based DCMs aredeveloped but they are not satisfactory since they are not capable ofcompensating the dispersion slope of the fiber used fortelecommunication (see Reference 1, Christopher R. Doerr, Opticalcompensation of system impairments, 5-10 Mar. 2006, OFC 2006).

FIG. 1 shows an optical pulse before entering optical fiber 101 (100),after exiting optical fiber 101 and before entering dispersioncompensator 102 (120), and after exiting dispersion compensator 102(140). An optical pulse consists of different chromatic components. Whentraveling through a medium e.g. a single mode optical fiber (referringto 101 in FIG. 1), the different components of light have differentspeeds, so these chromatic components get dispersed after traveling adistance, and the extent of the dispersion is proportional to thetraveling distance of the medium (referring to 120 in FIG. 1). Thedispersion of different chromatic components causes “distortion” of theshape of the optical pulse thus degrades the transmission performance ofthe digital networks. Compensation of the chromatic dispersion canrestore the “distorted” optical pulse thus improving the transmissionperformance of the digital networks.

Future optical networks need reconfigurability, this is because thetraveling distance needed for the optical pulse may vary with differentnetwork configurations, as a result the amount of the dispersion to becompensated should also be reconfigurable, or, tunable.

Particularly, FIG. 2 is a schematic diagram showing the usage of TunableChromatic Dispersion Compensator (TCDC) in an agile optical network. Asshown in FIG. 2, when changing network transmission configuration from afirst configuration A to B to a second configuration A to C by theoptical router 201, transmission distance also changes from A-B to A-C,in this case the Tunable Chromatic Dispersion Compensators (TCDCs) 202are needed to realize reconfigurability.

FIG. 3 shows a schematic overall diagram of the Tunable ChromaticDispersion Compensator (TCDC) 202. The single channel TCDC 202 mainlyconsists of a dispersive grating 301, a telescope structure (302 and303), a single mode waveguide 304 and a tangential coupling grating 305.The dispersive grating 301 disperses the light to a small angle, thetelescope structure, which consists of an object lens 302 and an eyepiece 303, can magnify this angle by a factor of f_(o)/f_(e), wheref_(o) and f_(e) are the focal lengths of the object lens and eye piece303 respectively. Light exiting the eye piece 303 will be coupled to thearc single mode waveguide 304 by the arc tangential coupling grating305, thus different colors experience different delay after exiting thepigtail, so dispersion is compensated. If the radius of the arc is r,dispersed angle is 2θ, then the maximum dispersion distance able to becompensated is 2rθ.

As discussed before, the “maximum compensated dispersion distance” is:

${2r\; \theta} = {2r \times {\arctan \left( {{\tan (\alpha)}\frac{f_{o}}{f_{e}}} \right)}}$

The amount of the compensated dispersion can be tuned by changing themagnification factor of the telescope structure (302 and 303) as shownin FIG. 3. The focal length of the object lens 302 is fixed in thissolution, so a deformable mirror with a variable focal length can beused for the eye piece 303. A kind of deformable mirror usingpiezo-electric actuators is proposed in a publication by Chris R. Doerr(see Reference 2, C. R. Doerr, et. al., 40-Gb/s colorless tunabledispersion compensator with 1000-ps/nm tuning range employing a planarlightwave circuit and a deformable mirror, 6-10 Mar. 2005, OFC 2005).

The present invention is intended to overcome the above problems for“tunable DCMs” for future dynamically reconfigurable optical networks.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an eye piece and atunable chromatic dispersion compensator using the same which are usedfor optical networking to overcome the above problems for “tunable DCMs”for future dynamically reconfigurable optical networks.

According to a first aspect of the present invention, an eye piece foruse in a tunable chromatic dispersion compensator is proposed, whichcomprises: a first strip made of a first metal; a second strip made of asecond metal, which is attached to the first strip; a heater/coolerwhich is attached to the second strip for heating/cooling the first andsecond strips; and a tunable positioning bar which is attached to theheater/cooler for keeping the position of the eye piece in the tunablechromatic dispersion compensator, wherein the first metal and the secondmetal have different expanding coefficients from each other, so that inresponse to a change in temperature, the shape of the eye piece ischanged from a first shape to a second shape.

Preferably, the first shape is a concave shape the second shape being aconvex shape; or the first shape is convex shape the second shape beinga concave shape.

Preferably, the first metal has an expanding coefficient smaller thanthat of the second metal. Alternatively, the first metal has anexpanding coefficient larger than that of the second metal.

Preferably, the first metal is suitable to reflect light within awavelength range for which the chromatic dispersion is to becompensated. Alternatively, the eye piece further comprises a reflectionfilm which is attached onto the first strip and suitable to reflectlight within a wavelength range for which the chromatic dispersion is tobe compensated.

Preferably, the heater/cooler is a thermal electric cooler.

Preferably, the tunable positioning bar is a piezoelectric transducer oris made of a third metal with large thermal expanding coefficient. Morepreferably, the third metal is the same as the second metal.

According to a second aspect of the present invention, a tunablechromatic dispersion compensator is proposed, which comprises an eyepiece according to the present invention.

Preferably, the tunable chromatic dispersion compensator furthercomprises: an object lens; an arc tangential coupler; an arc single modewaveguide; and a dispersive grating; wherein inputted light is dispersedto the object lens at the dispersive grating, a focal plane of theobject lens is overlapped with the eye piece, the eye piece is locatedsubstantially at the center of the arc tangential coupler, and the arcsingle mode waveguide is parallel to the arc tangential coupler, and thearc tangential coupler couples the light tangentially into the arcsingle mode waveguide.

Preferably, the tunable chromatic dispersion compensator furthercomprises a coarse grating, and the dispersive grating is formed by achannel grating array, so that the inputted light is firstly dispersedaccording to different channels at the coarse grating and then projectedonto the channel grating array.

Preferably, the tunable chromatic dispersion compensator furthercomprises a heater/cooler which is attached onto the channel gratingarray and is controlled to produce a temperature distribution desiredfor compensating the chromatic dispersion slope along the channelgrating array. More preferably, the temperature distribution is a linearincreasing/decreasing temperature distribution along the channelgrating.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be clearer from the following detailed description aboutthe non-limited embodiments of the present invention taken inconjunction with the accompanied drawings, in which:

FIG. 1, which has been described, is a schematic diagram showing thedispersed and restored optical pulses;

FIG. 2, which has been described, is a schematic diagram showing theusage of TCDC in an agile optical network;

FIG. 3, which has been described, shows a schematic overall diagram ofthe Tunable Chromatic Dispersion Compensator (TCDC);

FIG. 4 shows a tunable eye piece provided according to the presentinvention in a telescope;

FIG. 5 shows the tangential coupling of the dispersed light from the eyepiece (of the telescope) to the single mode waveguide;

FIG. 6 shows a schematic overall diagram of a Multi-channel TCDC;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, the present invention will be described in accordance withthe drawings. In the following description, some particular embodimentsare used for the purpose of description only, which shall not beunderstood as any limitation to the present invention but the examplesthereof. While it may blur the understanding of the present invention,the conventional structure or construction will be omitted.

FIG. 4 shows a tunable eye piece 403 provided according to the presentinvention in a telescope. As shown in FIG. 4, an eye piece 403 is aconcave mirror 4031 made of metal A, and is attached onto a strip 4032made of metal B, the metal A and metal B have different thermalexpanding coefficients. For example, the metal A may have a smallerthermal expanding coefficient, and the metal B may have a larger thermalexpanding coefficient. For example, the metal A is iron (Fe) and themetal B is copper (Cu), so that when the eye piece 403 is heated, itwill be changed from a convex mirror into a concave mirror, and when theeye piece 403 is cooled, it will be changed from a concave mirror into aconvex mirror. Alternatively, the metal A may have a larger thermalexpanding coefficient, and the metal B may have a smaller thermalexpanding coefficient, such as, copper (Cu) for use as the metal A andiron (Fe) for use for the metal B, so that when the eye piece 403 isheated, it will be changed from a concave mirror into a convex mirror,and when the eye piece 403 is cooled, it will be changed from a convexmirror into a concave mirror. Furthermore, it shall be noticed that themetal A (e.g. gold (Au)) or a reflection film (e.g. a gold (Au) film)(not shown) formed thereon is suitable to reflect the light within thewavelength range for which the chromatic dispersion is to becompensated. A TEC (thermal electric cooler) 4033 is attached onto metalstripe B on the opposite side opposite to the metal A. When heated orcooled by the TEC 4033, the focal length of (or, half radius of) theconcave mirror will vary due to the different expansions of the metalstrip A and metal strip B. When the eye piece 403 is changed from aconcave mirror to a convex mirror, the sign of the compensateddispersion will change from positive to negative. A tunable positioningbar 4034 is attached onto the eye piece, which length can be varied byelectrical means e.g. PZT (piezoelectric transducer) or thermal meanse.g. metal with large thermal expanding coefficient in combination witha TEC. The purpose of the tunable positioning bar 4034 is to keep thetelescope structure (302 and 303) as shown in FIG. 3 well tuned, or, inother words, to keep the focal planes of the object lens 302 and the eyepiece 303 well overlapped while the focal length of the eye piece 303 isbeing changed.

FIG. 5 shows the eye piece 503 and the Planar Lightwave Circuit (PLC)devices (5001-5005), in which the dispersed light from the eye piece 503(of the telescope) is tangentially coupled to the single mode waveguide5004. The tangential coupling grating 5003 is designed to couple thelight tangentially, or, in other words, to output light normal to theincident light, so the light exiting the grating 5003 will enter thesingle mode waveguide 5004 which is parallel to the grating 5003 itself.This kind of grating coupler is reported to have a coupling efficiencyof 60-70% (see References 3 and 4: F. Van Laere, et. al., Compactgrating couplers between optical fibers and Silicon-on-Insulatorphotonic wire waveguides with 69% coupling efficiency, 5-10 Mar. 2006,OFC 2006; and Bin Wang, et. al., Compact slanted grating couplers, 26Jul. 2004, Vol. 12, No. 15, Optics Express 3313). Additionally, therelative positioning of the eye piece 503 with respect to the arctangential coupling grating 5003 as shown in FIG. 5 is that the eyepiece 503 is located substantially at the center of the arc, that is, atthe radius “r” away from the arc tangential coupling grating 5003, allthe time.

FIG. 6 shows a schematic overall diagram of a Multi-channel TCDC. Athree-channel TCDC 600 is depicted in FIG. 6. The channel count can bebigger like 40 or more. Compared to the single channel TCDC 202, thedifference of a multi-channel TCDC 600 is in that it utilizes a “coarsegrating 6001” to disperse the channels from the input light, and thechannels will be projected onto a grating array 601 instead of just onegrating 301 as in the single channel TCDC 202. The remaining portion ofthe multi-channel TCDC 600 shares the same design with the singlechannel TCDC 202. All the gratings on the grating array 601 can diffractthe center wavelengths of the channels in parallel. Thus all thechannels share the same image area on the focal plane of the telescopestructure (302 and 303). The TECs 6002 on the channel grating array 601can be controlled to produce a desired temperature distribution(gradient), for example, a linear increasing/decreasing temperaturedistribution, along the grating array 601 which can vary the gratingperiods to get the slope compensation as needed in telecom systems.

Therefore, according to the present invention, the inventive TCDC mayachieve the following technical advantages.

1. Tunable: the amount of the compensated dispersion can be tuned byadjusting the focal length of the eye piece via TEC attached to it. Forexample, if 2θ=PI/4 and r=200 mm, then the dispersion length isL=2rθ=2×200 mm×π/4=0.314 m, assume the velocity of the light in thewaveguide is V=2×10⁸ m/s and the effective bandwidth is B=0.5 nm, thenthe amount of the compensated dispersion isCD=L/(B×V)=0.314/(0.5×2×108)=3140 ps/nm, the sign of the compensateddispersion is decided by the eye piece, when the eye piece is a concavemirror, the sign is plus, when the eye piece is convex mirror, the signis minus.2. Multi-channel operating: suitable for dispersion compensation forDWDM system e.g. an 80-grating array can be designed for an 80 channelDWDM system.3. Slope compensation, the dispersion slope can be compensated byintroducing a specific temperature distribution (gradient) along thechannel grating array, this feature makes the invention more attractiveand practical since none of other tunable dispersion compensation methodis capable of compensating the dispersion slope according to Reference 1(Christopher R. Doerr, Optical compensation of system impairments, 5-10Mar. 2006, OFC 2006).4. Wide-band, cascadability: unlike other compensation solutions, thepresent invention has very wide pass-band so can be cascaded in longoptical links.

The above embodiments are provided for the purpose of example only, andare not intended to limit the present invention. It is to be understoodby those skilled in the art that there may be various modifications orreplacements to the embodiments without departing from the scope and thespirit of the present invention, and they shall fall into the scopedefined by the appended claims.

REFERENCE LIST

-   Reference 1: Christopher R. Doerr, Optical compensation of system    impairments, 5-10 Mar. 2006, OFC 2006;-   Reference 2: C. R. Doerr, et. al., 40-Gb/s colorless tunable    dispersion compensator with 1000-ps/nm tuning range employing a    planar lightwave circuit and a deformable mirror, 6-10 Mar. 2005,    OFC 2005;-   Reference 3: F. Van Laere, et. al., Compact grating couplers between    optical fibers and Silicon-on-Insulator photonic wire waveguides    with 69% coupling efficiency, 5-10 Mar. 2006, OFC 2006;-   Reference 4: Bin Wang, et. al., Compact slanted grating couplers, 26    Jul. 2004, Vol. 12, No. 15, Optics Express 3313.

1. An eye piece for use in a tunable chromatic dispersion compensator,comprising: a first strip made of a first metal; a second strip made ofa second metal, which is attached to the first strip; a heater/coolerwhich is attached to the second strip for heating/cooling the first andsecond strips; and a tunable positioning bar which is attached to theheater/cooler for keeping the position of the eye piece in the tunablechromatic dispersion compensator, wherein the first metal and the secondmetal have different expanding coefficients from each other, so that inresponse to a change in temperature, the shape of the eye piece ischanged from a first shape to a second shape.
 2. The eyepiece of claim1, wherein the first shape is a concave shape the second shape being aconvex shape; or the first shape is convex shape the second shape beinga concave shape.
 3. The eye piece for use in the tunable chromaticdispersion compensator according to claim 1, wherein the first metal hasan expanding coefficient smaller than that of the second metal.
 4. Theeye piece for use in the tunable chromatic dispersion compensatoraccording to claim 1, wherein the first metal has an expandingcoefficient larger than that of the second metal.
 5. The eye piece foruse in the tunable chromatic dispersion compensator according to claim1, wherein the first metal is suitable to reflect light within awavelength range for which the chromatic dispersion is to becompensated.
 6. The eye piece for use in the tunable chromaticdispersion compensator according to claim 1, wherein it furthercomprises a reflection film which is attached onto the first strip andsuitable to reflect light within a wavelength range for which thechromatic dispersion is to be compensated.
 7. The eye piece for use inthe tunable chromatic dispersion compensator according to claim 1,wherein the heater/cooler is a thermal electric cooler.
 8. The eye piecefor use in the tunable chromatic dispersion compensator according toclaim 1, wherein the tunable positioning bar is a piezoelectrictransducer or is made of a third metal with large thermal expandingcoefficient.
 9. The eye piece for use in the tunable chromaticdispersion compensator according to claim 8, wherein the third metal isthe same as the second metal.
 10. A tunable chromatic dispersioncompensator, comprising: an eye piece according to claim
 1. 11. Thetunable chromatic dispersion compensator according to claim 10 furthercomprising: an object lens; an arc tangential coupler; an arc singlemode waveguide; and a dispersive grating; wherein inputted light isdispersed to the object lens at the dispersive grating, a focal plane ofthe object lens is overlapped with the eye piece, the eye piece islocated substantially at the center of the arc tangential coupler, andthe arc single mode waveguide is parallel to the arc tangential coupler,and the arc tangential coupler couples the light tangentially into thearc single mode waveguide.
 12. The tunable chromatic dispersioncompensator according to claim 11, wherein it further comprises a coarsegrating, and the dispersive grating is formed by a channel gratingarray, so that the inputted light is firstly dispersed according todifferent channels at the coarse grating and then projected onto thechannel grating array.
 13. The tunable chromatic dispersion compensatoraccording to claim 12, wherein it further comprises a heater/coolerwhich is attached onto the channel grating array and is controlled toproduce a temperature distribution desired for compensating thechromatic dispersion slope along the channel grating array.
 14. Thetunable chromatic dispersion compensator according to claim 13, whereinthe temperature distribution is a linear increasing/decreasingtemperature distribution along the channel grating.